This invention relates to an improvement of a swirl stream type jet mill with a rotary sorter or classifier, and more particularly to a micromilling apparatus improved in micromilling power consumption and in milled particle size distribution.
In general, a swirl stream type jet mill with a rotary classifier or sorter (hereinafter referred to as "an internal classification type jet mill", when applicable) operates as follows: Compressed air is jetted from micromilling nozzles to form high speed air streams, to cause particles to collide with one another, thereby to mill solid materials. In order to obtain particles having a target particle size, the particles thus processed are classified by the centrifugal force provided by the rotary classifier.
The internal classification type jet mill is advantageous in the following points: That is, since the compressed air is jetted in the above-described manner, the lowering of temperature due to its adiabatic expansion effect is caused. This phenomenon makes it possible to mill a solid material which should not be heated. In the internal classification type jet mill, the classifier is provided inside the swirl stream type jet mill. Therefore, when compared with an ordinary closed circuit system (in which the classifier is provided outside the swirl stream type jet mill), the internal classification type jet mill is smaller in the number of components, and is able to handle different kinds of particles with ease, and can readily be cleaned. In addition, in the internal classification type jet mill, collision of particles, i.e., surface milling is utilized. Therefore, the internal classification type jet mill is suitable for milling a material into ultrafine particles.
The above-described internal classification type jet mill suffers from the following difficulties: The jet mill uses a large quantity of compressed air. Accordingly, it needs a large capacity compressor. Hence, the jet mill is two times to five times greater in micromilling energy consumption than a mechanical mill. Furthermore, the jet mill utilizes collision of particles as was described above, and accordingly it is wide in milled particle distribution.
A milling machine disclosed in Japanese Patent Application (OPI) No. 319067/1988 (the term "OPI" as used herein means an "unexamined published application") is an example of the internal classification type jet mill. Normally, the speed of a swirl stream formed by the jet air is higher than the speed of rotation of the sorting rotor. Hence, in the case where the sorting rotor is set near the field of swirl streams, the effect of classification is not so high. The milling machine is still great in milling energy consumption because it is a jet mill using a compressor.